1. Field of the Invention
The present invention relates to an illuminating device which has a light guide body and a blurring structure used to control illuminating light paths, and a display device.
2. Description of Related Art
Large-size liquid crystal televisions and flat display panels in recent years mainly adopt illuminating devices of a direct-lit type, and illuminating devices of an edge-lit type. With the illuminating device of the direct-lit type, a plurality of cold-cathode tubes which are light sources and LEDs (light emitting diodes) are regularly arranged in the back surface of the panel. A diffusing plate with high light scattering performance is used between an image display element such as a liquid panel and light sources, such that cold-cathode tubes which are light sources and LEDs are not seen.
By contrast with this, with the illuminating device of the edge-lit type, a plurality of cold-cathode tubes and LEDs are arranged on end surfaces of a translucent plate which is referred to as “light guide plate”. Generally, on a surface (light deflection surface) on the opposite side of an emission surface of the light guide plate (a surface opposing to an image display element), light deflection elements are formed which efficiently guide incident lights which are incident from the end surfaces of the light guide plate, to the emission surface. Various light deflection elements formed on the light deflection surface are proposed including light deflection elements which are formed by printing a white dot pattern and light deflection elements which have lens shapes in order to efficiently guide light to the emission surface.
However, with the edge-lit type, the number of light sources to be disposed is limited due to a structure in which light sources are arranged only on the end surfaces of the light guide plate. Hence, when a liquid crystal display device becomes larger, it is difficult to brighten the entire display, and therefore an optical sheet which enhances the brightness plays an important role.
As means for enhancing the brightness of the liquid crystal display screen, a brightness enhancement film (BEF) which is the registered trademark of 3M company in the U.S.A. is widely used as a lens sheet.
FIG. 28 to FIG. 30 illustrate brightness enhancement films disclosed in Japanese Patent Application Publication No. 1-37801 and Japanese Patent Application Laid-Open No. 6-102506. In FIG. 28, a planar light source 182, a BEF 185 which is a brightness enhancement film on which light emitted from the light source 182 is incident, and a liquid crystal panel 184 are schematically disposed. As illustrated in FIG. 18, the BEF 185 is an optical film in which unit prisms 187 having a sectional triangular shape are aligned on a transparent base material 186 periodically in one direction. These unit prisms 187 are formed in a larger size (pitch) than the wavelength of light.
The BEF 185 can condense light from “off-axis”, and redirect or “recycle” this light “on-axis” toward the viewer. That is, the BEF 185 can increase an on-axis brightness by decreasing an off-axis brightness when a liquid crystal display device is used (viewed). “On-axis” refers to a direction matching with a field-of-view direction F′ of the viewer in FIG. 28, and generally refers to a normal direction side with respect to the display screen of the liquid crystal panel 184.
Further, when a lens sheet represented by the BEF 185 is used, it is possible to prevent unevenness of lights emitted from the light guide plate by arranging a diffusion film to which a diffusion filler is coated on the transparent base material, between the light guide plate and lens sheet.
Furthermore, when the diffusion film is arranged between the lens sheet and liquid crystal panel, it is possible to reduce side lobes of emission lights caused by the prism sheet, and prevent a moire interference pattern produced between the regularly aligned lenses and liquid crystal pixels.
By the way, the light guide plate used for the edge-lit type has a light deflection surface opposite to the emission surface as described above and, on the light deflection surface, light deflection elements of a white dot pattern, microlenses (concave type or convex type) and other lens shapes are formed.
However, all light deflection elements are formed with a reflection surface or structure which is regularly or pseudo-irregularly aligned, and therefore have a problem of the above interference (moire interference pattern) with a lens sheet represented by the BEF 102, and a problem that light deflection elements on the light deflection surface are seen through and viewed as unevenness of the brightness. As a solution for these problems, a method of using a diffusion film disclosed in Japanese Patent Application Laid-Open No. 2004-295080 is generally used between the light guide plate and lens sheet.
Further, although the BEF 185 is one of the most efficient lens sheet which enhances the brightness in the front surface direction, light condensing performance of the BEF 185 alone is not enough to provide a sufficient brightness for middle-size or large-size liquid crystal display devices over 20 inches. Although, for example, one method of arranging two BEFs 185 in a crossed manner may be used as a method of further enhancing the brightness of a liquid crystal display device, there is a problem that the view angle of the liquid crystal display device becomes extremely narrow. Compared to, for example, notebook computers and mobile information terminals, liquid crystal display devices for televisions require sufficient view angles because the liquid crystal display devices are viewed from directions other than the front direction, and therefore a sufficient view angle is required in a screen horizontal direction in particular.
Hence, there is a problem that a diffusion film with little light condensing performance needs to be arranged to cover light deflection elements formed on the light deflection surface such that the light deflection elements are not seen through, and a problem that a required brightness cannot be provided in the liquid crystal display device with a configuration using one BEF 185 as an optical sheet having light condensing performance.
By the way, as means for covering light deflection elements without using the above diffusion film, Japanese Patent Application Laid-Open No. 6-265732 and Japanese Patent Application Laid-Open No. 6-281934 disclose means for covering light deflection elements by inclining the prism sheet with respect to a direction in which the light deflection elements are aligned. According to Japanese Patent Application Laid-Open No. 6-265732, a configuration is employed where the prism sheet is inclined in a range equal to or more than 15 degrees and equal to or less than 75 degrees with respect to a direction in which the light deflection elements are aligned, and the prism sheet is inclined in a range equal to or more than 15 degrees and equal to or less than 75 degrees with respect to pixels of the liquid crystal display device. Further, according to Japanese Patent Application Laid-Open No. 6-281934, a configuration is employed where the direction in which the light deflection elements are aligned and the top of the prism sheet cross in a range equal to or more than 10 degrees and equal to or less than 70 degrees.
However, although Japanese Patent Application Laid-Open No. 6-265732 discloses that, when the prism sheet is inclined in a range equal to or more than 15 degrees and equal to or more than 75 degrees, particularly, at 63.5 degrees or 26.5 degrees of the optimal angle, with respect to the pixels of the liquid crystal display device, the covering performance for the light deflection elements is optimal, the prism sheet is inclined at a great angle with respect to pixel alignment of the liquid crystal display device. The inclination of the prism sheet with respect to pixel alignment of the liquid crystal display device means that the prism sheet inclines in the vertical and horizontal directions of the screen, and therefore there is a problem that the display brightness of the liquid crystal display device is distorted vertically asymmetrically or bilaterally asymmetrically. Further, there is also a problem that the front surface brightness decreases compared to a case where the prism sheet is not inclined.
By contrast with this, Japanese Patent Application Laid-Open No. 6-281934 discloses that, in case of hexagonal arrangement in which a shape connecting the most adjacent light deflection elements of the light guide plate forms a regular triangular shape, the line (virtual line on the light guide plate) connecting the most adjacent light deflection elements and the linear top of the lens sheet preferably cross at 30 degrees. For example, referring to FIG. 6 of Japanese Patent Application Laid-Open No. 6-281934, the direction of the linear top of the lens sheet includes three types of the directions of the ±30 degree with respect to the vertical direction and horizontal direction. However, the experiment conducted by the inventors of the present invention found that, when the linear top of the lens sheet is in the horizontal direction with the configuration in Japanese Patent Application Laid-Open No. 6-281934, sufficient conversing performance cannot be provided, and, further, when the linear top of the lens sheet is in a direction at ±30 degrees with respect to the vertical direction as described above, the display brightness of the liquid crystal display is distorted as described above and the front surface brightness decreases.
Further, light guide plates for liquid crystal televisions are made thinner in recent years and intervals to arrange light deflection elements are becoming wider, and therefore the configurations disclosed in Japanese Patent Application Laid-Open No. 6-265732 and Japanese Patent Application Laid-Open No. 6-281934 have a problem that covering performance is not sufficient and unevenness of the brightness caused by the light deflection elements are seen on the screen. Hence, an illuminating device of the edge-lit type is demanded to have an optical sheet having high light condensing performance and high covering performance of preventing light deflection elements from being seen, and a liquid crystal display device is demanded to have the illuminating device which provides a high brightness and a wide horizontal view angle without distorting the display brightness.